The composition of a plant cell wall is complex and variable and contains several carbohydrate biopolymers. Polysaccharides are mainly found in the form of long chains of cellulose (the main structural component of the plant cell wall), hemicellulose (comprising various β-xylan chains, such as xyloglucans) pectin and lignin. The most abundant hemicelluloses are xylans and their derivatives such as arabinoxylan and xyloglycan.
Plant hemicelluloses include xylan, arabinoxylan, glucuronoarabinoxylan and xyloglucan. Xylan (CAS Registry No. 9014-63-5) consists of a backbone of β-1,4-linked D-xylopyranosyl units, optionally substituted with side chains such as arabinose and/or glucuronic acid residues. The structure is:
→4)-β-D-Xylp-(1→4)-β-D-Xylp(2←1A)-(1→4)-β-D-Xylp-(1→4)-β-D-Xylp(3←1B)-(1→
(Xy1p=xylopyranosyl unit; A=α-(4-O)-methyl-(D-glucuronopyranosyl) unit, sometime an acetyl; and B=α-(L-arabinofuranosyl) unit, sometimes an acetyl).
Xylans may represent more than 30% of the dry weight of terrestrial plants. Hence xylan is an important component of materials from natural sources that are used in industrial processes ranging from baking, improvement of animal feed conversion and paper production.
Basic differences exist between monocotyledons (e.g. cereals and grasses) and dicotyledons (e.g. clover, rapeseed and soybean) and between the seed and vegetative parts of the plant. Monocotyledons are characterized by the presence of an arabinoxylan complex as the major hemicellulose backbone, and the main structure of hemicellulose in dicotyledons is a xyloglucan complex. Higher pectin concentrations are found in dicotyledons than in monocotyledons. Seeds are generally high in peptic substances but relatively low in cellulosic material.
Cellulose degrading enzymes are used for the processing of plant material in food as well as feed applications or as a food or feed additive due to of their capability to act on main plant cell wall substituents.
Most of the cellulose degrading enzymes available to the industry appear to be xylanases with a relatively low molecular weight and a moderate stability at higher temperatures. However, for certain applications it is desirable to use a xylanase with a relatively high thermostability. If a xylanase is to be used as an animal feed additive then a high thermostability is preferred because of the high temperature conditions applied during pelleting the animal feed.